Because of its expression in a large percentage of melanoma lesions and its restricted distribution in normal tissues, the human high molecular weight-melanoma associated antigen (HMW-MAA) has been used for immunotherapy in patients with melanoma. The proposed studies stem from our previous findings that i) unresponsiveness to HMW-MAA, which is a self-antigen, can be overcome by immunizing patients with melanoma with the mouse anti-idiotypic (anti-id) mAbMK2-23, which mimics the determinant, defined by anti-HMW-MAA mAb 763.74; ii) anti-HMW-MAA antibodies elicited by mAb MK2-23 appear to have a beneficial effect on the clinical course of the disease, although they have a low titer and iii) the apparent beneficial effect of anti-HMW-MAA antibodies on the disease does not appear to be mediated by complement- and/or cell-dependent lysis of melanoma cells, but is likely to reflect an effect of the antibodies on the biology of melanoma cells. It is our working hypothesis that enhancement of the humoral anti-HMW-MAA immunity may increase its beneficial effect on the clinical course of melanoma. Since the low titer of anti-HMW-MAA antibodies elicited by anti-id mAb MK2-23 does not reflect its low immunogenicity, but its low mimicry of HMW-MAA, we plan to test the hypothesis that improvement of the HMW-MAA mimicry may enhance the humoral anti-HMW-MAA immune response. To test our hypothesis we will utilize the 12 amino acid long peptide P763.74 which has been isolated from the LX-8 phage display peptide library with anti-HMW-MAA mAb 763.74. Like the anti-id mAb MK2-23, the peptide P763.74 has been found to be able to break unresponsiveness to HMW-MAA in rabbits with a constitutive expression of a human HMW-MAA homologue and to induce low titer HMW-MAA specific antibodies. To improve the mimicry of HMW-MAA amino acid substitutions will be introduced in the peptide mimic P763.74 on the basis of the characterization at the atomic level of its interactions with mAb 763.74. The modified peptides will be tested in their in vitro reactivity with mAb 763.74 and for their ability to induce HMW-MAA specific antibodies in hosts with a constitutive HMW-MAA expression. Since anti-HMW-MAA antibodies are poor in mediating complement -and cell- dependent lysis of melanoma cells, we plan to investigate the mechanisms underlying the direct effect of anti-HMW-MAA antibodies on the biology of melanoma.